Jobs said on Wednesday that he could no longer be chief executive of the company he co-founded. He had gone on medical leave in January for an undisclosed condition after years of fighting a rare type of pancreatic cancer and other health issues.

The type of pancreatic cancer is caused by an islet cell neuroendocrine tumor.

Jobs was reported to have undergone a liver transplant in 2009 to fight off the spread of the neuroendocrine tumor. The procedure is experimental and is fraught with complications.

Jobs has never publicly stated the reason for his liver transplant.

Dr. Simon Lo, director of pancreatic and biliary diseases at Cedars-Sinai Medical Center in Los Angeles said the most likely serious complication after Jobs' liver transplant would have been further spread of the cancer, which could have forced Jobs to leave his position permanently. Lo has not treated Jobs.

As many as 80 percent of patients who get liver transplants to treat this type of cancer live for at least five years, according to the University of California San Francisco.

Lo said a recent study showed about three-quarters of patients who got a liver transplant because of cancer saw their cancer return within two to five years. The cancer can return to the liver or spread to other organs in the body.

The immunosuppressant drugs necessary for a liver transplant also make it harder for the body to fight the return of the disease.

Jobs may be "confronting both the liver transplant related specific problems, as well as the cancer itself," Lo said. "Whenever you put patients on immunosuppressant medications, there's always a risk that it could take away natural resistance, so the cancer could grow faster."

Although this cancer is broadly lumped in with pancreatic cancer, neuroendocrine tumors have a different nature from most pancreatic tumors, which are highly lethal and which kill 95 percent of patients within five years.

Neuroendocrine tumors are more easily treated and less aggressive. According to the National Cancer Institute, there are only 200 to 1,000 new cases a year.

Jobs had a neuroendocrine tumor removed in 2004 and said afterward all the cancer was gone, and that he did not require chemotherapy or radiation treatment.

But he remained noticeably thin, even gaunt, and took time off in 2009 to deal with what he initially termed a hormone imbalance, again giving few details.

Islet-cell tumors can cause over-secretion of hormones, including insulin, into the bloodstream, wreaking havoc on digestion and leading to drastic weight loss. They are usually easily removed surgically, but recur in roughly half of patients, doctors say, possibly spreading to other organs.

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